When a thermoplastic resin sheet is molded by melt extrusion, a cooling/shaping roll that has a fine grooved structure on its surface is often used to transfer said fine grooved structure onto the sheet surface, thereby imparting various functions onto the surface of the molded sheet made from the thermoplastic resin. For example, a roll having a fine prism structure on its surface is used to transfer said prism structure onto a sheet to manufacture a highly functional luminance enhancement sheet by melt extrusion molding.
In order to mold such a sheet by melt extrusion, generally, a sheet of a molten thermoplastic resin that flows out from a lip part of a T-die or a coat-hanger die is pressed between a cooling/shaping roll having a fine grooved structure on its surface and a press roll. In general, the thinner the sheet is, the more the transfer property of the fine grooved structure is likely to deteriorate. This is, for example, because a sheet-like molten resin is easily cooled in a region called “air gap” between the die lip and the roll pressing part (the part between the cooling/shaping roll and the press roll where pressing takes place), and also the sheet-like molten resin is solidified relatively fast at the roll pressing part due to heat transfer to the cooling/shaping roll.
In order to enhance the transfer property onto a thin sheet manufactured by melt extrusion molding, attempts such as raising the set temperatures of the die and the roll, and raising the pressing pressure have been made. However, if the set temperatures of the die and the roll are too high, the thermoplastic resin is not sufficiently solidified and cooled as the sheet is pressed between the cooling/shaping roll and the press roll and peeled off from the cooling/shaping roll. As a result, the molded sheet sticks to the cooling/shaping roll, causing an appearance defect called peel marks. Accordingly, there is a limit to raising the set temperatures of the die and the roll. In addition, if the pressing pressure is raised too much, roll bending occurs due to lack of roll rigidity which results in defects such as difficulty in controlling the film thickness of the molded sheet or difficulty in uniform transfer.
As a method for manufacturing a shape-transferred optical sheet by melt extrusion molding, Japanese Unexamined Patent Application Publication No. 2012-66410 describes a method comprising a step of laminating a first layer made of an easily peelable protection film and a second layer made of a film with an optical shape by a coextrusion method, wherein the first layer made of the protection film is made from a polyethylene-based resin or a polypropylene-based resin while the optical shape of the second layer is formed on the surface that does not make contact with the first layer made of the protection film.